三种方式使得iOS应用能够在后台进行数据更新和下载

首先在info plist文件中开启UIBackgroundModes的Background fetch。或者手动编辑这个值

<key>UIBackgroundModes</key><array>     <string>fetch</string></array>

iOS默认不进行background fetch，需要设置一个时间的间隔

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{     //UIApplicationBackgroundFetchIntervalMinimum表示尽可能频繁去获取，如果需要指定至少多少时间更新一次就需要给定一个时间值     [application setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];     return YES;}

最后在App Delegate里实现下面的方法，这个方法只能在30秒内完成。

- (void) application:(UIApplication *)applicationperformFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler{NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];NSURLSession *session = [NSURLSession sessionWithConfiguration:sessionConfiguration];NSURL *url = [[NSURL alloc] initWithString:@"http://yourserver.com/data.json"];NSURLSessionDataTask *task = [session dataTaskWithURL:urlcompletionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {if (error) {completionHandler(UIBackgroundFetchResultFailed);return;}// 解析响应/数据以决定新内容是否可用BOOL hasNewData = ...if (hasNewData) {completionHandler(UIBackgroundFetchResultNewData);} else {completionHandler(UIBackgroundFetchResultNoData);}}];// 开始任务[task resume];}

测试 

• 通过查看UIApplication的applicationState

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{     NSLog(@"Launched in background %d", UIApplicationStateBackground == application.applicationState);     return YES;}

Remote Notification 

在普通的远程通知里带上content-available标志就可以在通知用户同时在后台进行更新。通知结构如下

{     "aps" : {          "content-available" : 1     },     "content-id" : 42}

接收一条带有content-available的通知会调用下面的方法

- (void)application:(UIApplication *)applicationdidReceiveRemoteNotification:(NSDictionary *)userInfofetchCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler{NSLog(@"Remote Notification userInfo is %@", userInfo);NSNumber *contentID = userInfo[@"content-id"];// 根据 content ID 进行操作completionHandler(UIBackgroundFetchResultNewData);}

利用NSURLSession进行background transfer task 

使用[NSURLSessionConfiguration backgroundSessionConfiguration]创建一个后台任务，当应用退出后，崩溃或进程被关掉都还是会运行。

范例，先处理一条远程通知，并将NSURLSessionDownloadTask添加到后台传输服务队列。

- (NSURLSession *)backgroundURLSession{static NSURLSession *session = nil;static dispatch_once_t onceToken;dispatch_once(&onceToken, ^{NSString *identifier = @"io.objc.backgroundTransferExample";NSURLSessionConfiguration* sessionConfig = [NSURLSessionConfiguration backgroundSessionConfiguration:identifier];session = [NSURLSession sessionWithConfiguration:sessionConfigdelegate:selfdelegateQueue:[NSOperationQueue mainQueue]];});return session;}- (void) application:(UIApplication *)applicationdidReceiveRemoteNotification:(NSDictionary *)userInfofetchCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler{NSLog(@"Received remote notification with userInfo %@", userInfo);NSNumber *contentID = userInfo[@"content-id"];NSString *downloadURLString = [NSString stringWithFormat:@"http://yourserver.com/downloads/%d.mp3", [contentID intValue]];NSURL* downloadURL = [NSURL URLWithString:downloadURLString];NSURLRequest *request = [NSURLRequest requestWithURL:downloadURL];NSURLSessionDownloadTask *task = [[self backgroundURLSession] downloadTaskWithRequest:request];task.taskDescription = [NSString stringWithFormat:@"Podcast Episode %d", [contentID intValue]];//执行resume保证开始了任务[task resume];completionHandler(UIBackgroundFetchResultNewData);}

下载完成后调用NSURLSessionDownloadDelegate的委托方法，这些委托方法全部是必须实现的。了解所有类型session task的生命周期可以参考官方文档：  https://developer.apple.com/library/ios/documentation/cocoa/Conceptual/URLLoadingSystem/NSURLSessionConcepts/NSURLSessionConcepts.html#//apple_ref/doc/uid/10000165i-CH2-SW42

#Pragma Mark - NSURLSessionDownloadDelegate- (void) URLSession:(NSURLSession *)sessiondownloadTask:(NSURLSessionDownloadTask *)downloadTaskdidFinishDownloadingToURL:(NSURL *)location{NSLog(@"downloadTask:%@ didFinishDownloadingToURL:%@", downloadTask.taskDescription, location);// 必须用 NSFileManager 将文件复制到应用的存储中，因为临时文件在方法返回后会被删除// ...// 通知 UI 刷新}- (void) URLSession:(NSURLSession *)sessiondownloadTask:(NSURLSessionDownloadTask *)downloadTaskdidResumeAtOffset:(int64_t)fileOffsetexpectedTotalBytes:(int64_t)expectedTotalBytes{}- (void) URLSession:(NSURLSession *)sessiondownloadTask:(NSURLSessionDownloadTask *)downloadTaskdidWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWrittentotalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite{}

后台的任务完成后如果应用没有在前台运行，需要实现UIApplication的两个delegate让系统唤醒应用

- (void) application:(UIApplication *)applicationhandleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler{// 你必须重新建立一个后台 seesiong 的参照// 否则 NSURLSessionDownloadDelegate 和 NSURLSessionDelegate 方法会因为// 没有 对 session 的 delegate 设定而不会被调用。参见上面的 backgroundURLSessionNSURLSession *backgroundSession = [self backgroundURLSession];NSLog(@"Rejoining session with identifier %@ %@", identifier, backgroundSession);// 保存 completion handler 以在处理 session 事件后更新 UI[self addCompletionHandler:completionHandler forSession:identifier];}- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session{NSLog(@"Background URL session %@ finished events.", session);if (session.configuration.identifier) {// 调用在 -application:handleEventsForBackgroundURLSession: 中保存的 handler[self callCompletionHandlerForSession:session.configuration.identifier];}}- (void)addCompletionHandler:(CompletionHandlerType)handler forSession:(NSString *)identifier{if ([self.completionHandlerDictionary objectForKey:identifier]) {NSLog(@"Error: Got multiple handlers for a single session identifier. This should not happen.");}[self.completionHandlerDictionary setObject:handler forKey:identifier];}- (void)callCompletionHandlerForSession: (NSString *)identifier{CompletionHandlerType handler = [self.completionHandlerDictionary objectForKey: identifier];if (handler) {[self.completionHandlerDictionary removeObjectForKey: identifier];NSLog(@"Calling completion handler for session %@", identifier);handler();}}原文地址 http://www.cnblogs.com/pandas/p/4334871.html